1. Field of the Invention
The present invention relates to an electro-optical device, e.g. to an active liquid crystal electro-optical device, in particular, to a device provided with two complementary thin film insulated gate field effect transistors (hereinafter referred to as C/TFTs) having a structure of modified transfer gate MTG).
Also, the present invention relates to a method for driving an active electro-optical device, in particular, to a method for driving an active electro-optical device with clear gradation level in a digital mode.
2. Description of the Prior Art
An active liquid crystal electro-optical device utilizing TFT is conventionally know. In this device, an amorphous or polycrystalline semiconductor is used for TFT, while either one of conductive type alone is used for each picture element thereof. Namely, an N-channel TFT(referred to as NTFT) is generally linked to the picture element in series.
Since the dielectric constant in a direction parallel to a molecular axis of the liquid crystal composition provided between substrates is different from that in a direction perpendicular thereto due to the material property thereof, arrangement of the composition can easily be made in both directions, horizontally or vertically, to the outside electric field. By utilizing the anisotropy of dielectric constant, the amount of transmitted light or of dispersion thereof is controlled in a liquid crystal electro-optical device, so as to perform ON/OFF display.
FIG. 2 shows an electro-optic property of nematic liquid crystal. When the applied voltage is small, which is indicated by Va or a point A, the amount of transmitted light is approximately 0%, and at Vb or point B, it is approximately 20%, while at Vc or point C, it is approximately 70%, and at Vd or point D, it amounts to approximately 100%. Therefore, when the points A and D alone are used, two-graded display in black-and-white is possible, while, when the points B, C, or the points where electro-optical property (transmittance) rises in FIG. 2, are used, the display of intermediate gradation is possible.
As for the conventional electro-optical device utilizing TFTs, gradation display was performed by varying the voltage applied to a gate of the TFT or that applied between source and drain thereof, and controlling the voltage in an analogue mode.
Concerning the conventional method of gradation display in the electro-optical device utilizing TFT, an explanation will be made: an N-channel thin film transistor used for the conventional electro-optical device has the voltage-current characteristic as shown in FIG. 3, which shows the voltage-current characteristic of the N-channel thin film transistor utilizing amorphous silicon, and of that utilizing poly-silicon.
By controlling the voltage applied to a gate electrode of the thin film transistor having such characteristic in an analogue mode, drain current can be controlled and therefore strength of the electric field to be applied to the liquid crystal can be varied, whereby gradation display is possible.
In the case of an electro-optical device having picture elements of, for example, 640xc3x97400 dots, however, it is difficult to manufacture all 256,000 TFTs without variation in characteristics thereof. It is thought that 16 gradation levels are limit of the number of gradation levels of such electro-optical device having 640xc3x97400 picture elements in order to achieve productivity and yield required for practical process.
A gradation display may be performed by predetermining the value of gate voltage, while controlling only the turning of ON/OFF by gate voltage, and by variably controlling source or drain voltage. In this case, however, about 16 gradation levels are considered to be a limit, based on the fact that the characteristic are unstable. In an analogue mode of the gradation display control, clear display was difficult due to variation in characteristics of TFT.
Another method of gradation display using multiple frames is suggested. As shown in the outline indicated in FIG. 11, when a gradation display is to be performed using, for example 10 frames, by making two frames out of ten transparent, while the remainder of eight frames nontransparent, average 20% of transparency can be displayed at picture element A. A picture element B displays 70% of transparency on an average in the same manner, while a picture element C 50% of transparency on an average.
When such a display is carried out, however, since the number of frame is practically reduced thereby, flickering and display failure were generated. To solve the problem, the increasing of frame frequency, or the like, is suggested, whereas, the increase in power to be consumed in accordance with the increase in driving frequency, as well as the difficulty in the achievement of higher operation speed IC, indicated a limit of this method.
An object of the present invention is to provide a method of compensating the variation in characteristics of TFT by inputting a reference signal repeated in a certain cycle and having signal level which varies during duration of the reference signal, from a controller side, in order to clarify the level of applied voltage, and by controlling the time of connecting the reference signal to the TFT by digital value, and thereby controlling the voltage to be applied to the TFT, namely, to offer an electro-optical device by using complementary thin film transistors (C/TFTs) having a structure of modified transfer gate (MTG) that performs digital gradation display.
The method is characterized in displaying gradation in an electro-optical device using a display drive method that has a display timing in relation to a time F for writing one picture plane and a time (t) for writing in one picture plane, without changing the time F, by applying a reference signal that has voltage variation in a cycle that is equal to the time (t) to one of the signal lines that are used for drive and selection of a picture element, as well as a selection signal at a certain timing within the time (t), to the other signal line, thereby determining the voltage to be applied to a liquid crystal, and thereby actually applying the voltage to the picture element.
In addition, the method is also characterized in high speed control without being limited by several tens of MHz that was a limit of data transfer speed for a conventional CMOS, since the timing is not dependent upon the transfer of the data, but is processed at a part for signal process with a high speed clock being added to a driver IC itself that is put on the electro-optical device.
FIG. 1 shows a concrete drive waveform for driving the electro-optical device in accordance with the present invention. The electro-optical device has a circuit configuration equivalent to a circuit diagram having a 2xc3x972 matrix form shown in FIG. 4. Herein used is a half wave of a sine wave, as the reference signal waveform of varied voltage in a certain period of time as described supra. Sine waves 309, 310 are applied to VDD1303, VDD2304 that fall in a direction of a scanning line, while two-polarity (hereinafter referred to as bipolar) signals are applied to VGG1301, VGG2302 that fall in a direction of information line. Digital control is carried out by a timing of applying the bipolar signals.
Namely, the amount of charge to be accumulated at the point A as well as electric potential at the point A are determined by changing the timing for selecting the signal of varied voltage in a certain period of time, as shown in 309 and 310, and the size of the electric field to be applied to the picture element as well as to the liquid crystal is determined by defining a certain value for the electric potential 313 of a counter electrode.
The timing of applying the bipolar signal to gate signal lines such as VGG1, VGG2 is not determined by the transfer speed of the information signal, but is regulated by the. reference clock input to the driver IC that is directly connected to the electro-optical device in the present invention. Namely, in the case of an electro-optical device of 640xc3x97400 dots, drive frequency is approximately 20 MHz based on the limitation of CMOS, and, in order to calculate the number of gradation levels by utilizing this value, the drive frequency is to be indicated as a product of the number of scanning lines, the number of frames, the bipolar pulse, and the number of gradation levels, from which 20 MHz is divided by (400xc3x9760xc3x972), to obtain the number of gradation levels which is 416, and it is needless to say that a display having 832 gradation levels is possible by dividing the display screen into two.
The present invention is characterized in performing gradation display in a digital method, instead of employing the conventional analogue method of gradation display. To obtain the effect, in the case of an electro-optical device having picture elements of 640xc3x97400 dots, it is very difficult to eliminate the variation in characteristics for all the TFTs of 256,000, at the time of manufacturing, and, practically, in consideration of mass production and yield, 16 gradation display is supposed to be a limit, whereas, the method of compensating the variation in characteristics of TFTs is employed in the present invention by inputting a reference signal from a controller side, in order to clarify the level of applied voltage, and by controlling the timing of connecting the reference signal to TFT. by digital value, and thus controlling the voltage to be applied to TFT, which allows for clear digital gradation display.
Also, clear digital gradation display is possible without changing the number of frames for one picture plane, by defining two kinds of drive frequency, whereby the generation of flickering concomitant with the decrease in the number of frames can be prevented.
For example. when a gradation display is performed in a normal analogue mode for an electro-optical device, for which 256,000 pairs of TFTs of 640xc3x97400 dots are formed in 300 nm square, a 16-gradation display is an upper limit due to the variation in TFT characteristics of approximately xc2x110%. In the case of digital gradation display in accordance with the present invention, however, since the variation in characteristics of TFT devices can be compensated, a 256-gradation display is possible, and a various and subtle color display of as many as 16,777,216 kinds of colors are possible thereby. When a software such as a television image is to be projected on the screen, for example, the projection of a xe2x80x9crockxe2x80x9d scene of the same color requires subtle difference in colors due to the existence of slight recesses and the like thereon, and the 16-gradation display is not suitable for the display as close to the natural coloration ion as possible. However, the subtle variation in tone can be displayed by the gradation display in accordance with the present invention.